The present invention relates to the production of electrically conductive composites, and is particularly concerned with a process for conferring varying degrees of conductivity on electrically nonconductive porous structural materials, and the resulting electrically conductive composites.
In the past several years organic polymers have been discovered which have metallic properties, particularly electrical conductivity up to about 1000 ohm.sup.-1 cm.sup.-1. These polymers include doped polyacetylene and polypyrrole.
Although there are many potential applications for conducting polymers, their use has been limited by the fact that they are chemically unstable, have poor mechanical properties and/or are difficult to produce in suitable forms.
Polypyrrole which is chemically more stable, for example, than the doped polyacetylene, has the disadvantage of being very brittle. Thin free-standing polypyrrole films from about 10 .mu.m to 20 mil thick have been produced on electrodes by electrochemical polymerization. However, these prior art films are too thin and too brittle to be useful in most structurally related applications. Particularly, the brittleness of such conducting polymer films renders their handling for large area applications extremely difficult, if not impossible.
Polypyrrole is produced by electropolymerization as described by A. F. Diaz, et al., in an article entitled "Electrochemical Polymerization of Pyrrole" in the Journal of Chemical Society, Chemical Communications, 1979, page 635. N-substituted analogs of pyrrole such as N-methyl-pyrrole and N-phenylpyrrole have been used to form polymers as reported by A. F. Diaz, et al., in an article entitled "Electrochemistry of Conducting Polypyrrole Films" in the Journal of Electroanalytical Chemistry, 129, (1981) pages 115-132. The products produced in these processes are thin (from 20 .mu.m to 30 .mu.m) free-standing films in which anions from the electrolyte, such as tetrafluoborate and perchlorate, are used to dope the polymer and balance the cationic charge of the polymer backbone. However, the resultant polymer film is brittle and does not have the bulk and ductility needed to make the material useful in structural-related applications.
U.S. Pat. No. 4,401,545 to Naarmann, et al., discloses electrically conductive polypyrrole complexes with nitroaromatic anions as dopants, prepared by the anodic oxidation of a pyrrole in a polar solvent, in the presence of a salt of an acidic nitroaromatic compound. However, the thickness of the resulting electrochemically produced polypyrrole complexes is limited and the resulting films are also relatively brittle and hence also have limited structural applicability.
U.S. Pat. No. 4,394,304 to Wnek, discloses a method of forming a conductive polymer by impregnating a processable polymer such as polyethylene or polystyrene, with a Ziegler Natta catalyst, exposing the impregnated polymer to acetylene gas to form polyacetylene within a matrix of the initial polymer, and introducing a dopant, such as iodine, into the polyacetylene, to form a conductive polymer blend. However, a major disadvantage of the resulting conductive polymer blend is the fact that it is unstable when exposed to air or to water.
U.S. application Ser. No. 646,716, filed Sept. 4, 1984, now U.S. Pat. No. 4,582,575, patented Apr. 15, 1986, titled "Electrically Conductive Composites and Method of Preparation," of L. F. Warren, Jr., L. Maus and D. S. Klivans, and assigned to the same assignee as the present application, discloses electrically conductive composites comprising a dielectric porous substance, e.g., fiberglass fabric, and a pyrrole polymer deposited in the pores of such substance. The composites are produced by contacting the porous substance with an anode in an electrolytic cell containing an electrolyte comprising a pyrrole monomer and a substantially non-nucleophilic anion such as bisulfate, and passing an electric current through the cell, thus electrochemically precipitating a conductive pyrrole polymer in the pores of such substance.
An object of the present invention is to readily confer varying degrees of conductivity on porous structural materials.
Another object of the invention is to provide a procedure for obtaining an electrically conductive composite formed of a substantially non-conductive or dielectric porous substance and a conductive polypyrrole, which is stable and has good mechanical properties.
A still further object of the invention is the provision of a process for depositing a polypyrrole within a dielectric porous structural material to produce an electrically conductive composite.